1. Field of the Invention
The present invention relates to the field of mobile communication systems. More particularly, the present invention relates to a method for detecting the cause of a Radio Link Failure (RLF) or handover failure in mobile communication systems.
2. Description of the Related Art
Along with the development of communication technologies, the mobile communication system has evolved into a System Architecture Evolution (SAE) system. FIG. 1 is a schematic diagram illustrating a structure of the SAE system according to the related art. Referring to FIG. 1, the SAE system includes an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 101 and a core network containing a Mobile Management Entity (MME) 105 and a Serving Gateway (S-GW) 106. The E-UTRAN 101 is configured to connect a User Equipment (UE) to the core network, and includes more than one evolved Node B (eNB) 102 and more than one Home eNB (HeNB) 103, and further includes an optional HeNB GateWay (HeNB GW) 104. The MME 105 and the S-GW 106 may be integrated in one module or may be implemented separately. An eNB 102 is connected with another eNB 102 via an X2 interface, and is connected with the MME 105 and the S-GW 106 respectively via an S1 interface. A HeNB 103 is connected with the MME 105 and the S-GW 106 respectively via an S1 interface; or is connected with the optional HeNB GW 104 via an S1 interface, and the HeNB GW 104 is then connected with the MME 105 and the S-GW 106 respectively via an S1 interface.
At an early stage of SAE system deployment or during an SAE system operating stage, it should take a large number of human and material resources to optimize parameters of the SAE system, especially radio parameters, so as to guarantee good coverage and capacity, mobile robustness, load balance when moving, and sufficient access speed of the UE in the SAE system. To save the manual and material resources consumed during the SAE system operation process, a method for self-optimizing the SAE system is currently proposed. During a self-optimization process, configurations of the eNB or HeNB are optimized according to a current state of the SAE system. Both the eNB and the HeNB may both be referred to as an eNB for convenience in explanation in the following description of the method for self-optimizing the SAE system.
FIG. 2 is a schematic diagram illustrating a basic principle for self-optimizing a SAE system according to the related art. Referring to FIG. 2, after the eNB turns on power or accesses the SAE system, a self-configuring process can be started. The self-configuring process includes basic configuration 210 and initial radio parameter configuration 220 for the eNB. The basic configuration 210 for the eNB includes configuring an Internet Protocol (IP) address of the eNB and detecting Operation Administration and Maintenance (OAM) 211, authenticating between the eNB and the core network 212, detecting the HeNB GW that the eNB belongs to when the eNB is the HeNB 213, and downloading parameters of software and operations of the eNB for self-configuration 214. The initial radio parameter configuration 220 is implemented according to experience or simulation. Because performance of each eNB in the SAE system will be affected by the environment of an area where the eNB is located, the eNB initializes the radio parameter configuration according to the environment of the area where the eNB is located. Specifically, the eNB performs the initial configuration for a neighboring area list 221 and the initial configuration for the load balance 222. After the self-configuring process, many parameters configured by the eNB are not the optimized. Therefore, in order to increase the performance of the SAE system, the configuration of the eNB should be optimized or adjusted, which is also referred to as self-optimization of the mobile communication system. The optimization or adjustment of the configuration of the eNB may be implemented by the eNB under the control of the OAM in the background. Specifically, there may be a standardized interface between the eNB and the OAM, and the OAM will transmit a parameter to be optimized to the eNB (i.e., eNB or HeNB) via the standardized interface, and then the eNB optimizes the parameter in the self-configuration according to the parameter to be optimized. In addition, the configuration of the eNB can also be optimized or adjusted by the eNB itself, i.e., the eNB detects and obtains performance to be optimized, and then optimizes or adjusts its parameter corresponding to the performance. Optimizing or adjusting the configuration of the eNB 230 may include self-optimizing a neighboring area list 231, self-optimizing the coverage and capacity (not shown), self-optimizing the mobile robustness (not shown), self-optimizing the load balance 232, and self-optimizing a Random Access CHannel (RACH) parameter (not shown), etc.
Currently, SAE release 8 has proposed self-optimizing a neighboring area list. Release 9 has defined basic schemes for self-optimizing the coverage and capacity and self-optimizing the mobile robustness. Basic principles of self-optimizing in the defined schemes are that according to a difference between the last successful handover time of the UE and the time that the UE tries to re-establish a Radio Resource Control (RRC) connection, the eNB determines whether handover is too late or too early or is to a wrong cell, so as to perform the self-optimization.
However, in the above scheme for the self-optimization according to the related art, after the UE successfully re-establishes the RRC connection, it is possible to determine whether the handover is too late or is to a coverage leak according to UE measurement information obtained by the UE. However, after the UE fails in re-establishing the RRC connection (e.g., the eNB receives an RRC re-establishment request but fails during the re-establishment process), if an eNB to which the UE re-establishes the RRC connection transmits a Radio Link Failure (RLF) instruction to an eNB covering a cell where the RLF occurs, the cell where the RLF occurs cannot differentiate whether it is a coverage leak or a too late handover. If the eNB to which the UE re-establishes the RRC connection does not transmit the RLF instruction to the eNB covering the cell where the RLF occurs, the cell where the RLF occurs can not know the difference between the last successful handover time of the UE and the time that the UE tries to re-establish the RRC connection, and thus can not differentiate whether the handover is too late or too early or is to a wrong cell. However, even if the UE reports the difference to the network after accessing the network again, the network may still make a wrong determination because the difference between the time that the UE tries to re-establish a RRC connection and the time of the RLF/handover failure is large.
As can be seen from the above analysis, in the self-optimization by using the method for detecting the cause of the RLF or handover failure according to the related art, after it fails to re-establish a RRC connection, the analysis on the failure cause may be wrong if the network still performs the self-optimization based on the failure cause determined by whether the RRC connection is successfully re-established. Consequently, the self-optimization process will also be wrong and network performance will be affected.